1. Field of the Invention
The present invention relates to a valve timing adjusting apparatus for an internal combustion engine, wherein the valve timing adjusting apparatus adjusts valve timing of a valve that is opened and closed by a camshaft based on torque transmitted from a crankshaft.
2. Description of Related Art
Conventionally, a valve timing adjusting apparatus, which has a housing and a vane rotor, has been widely used. For example, the housing of the conventional valve timing adjusting apparatus is synchronously rotated with a crankshaft, and the vane rotor is synchronously rotated with a camshaft. In the above valve timing adjusting apparatus, vanes of the vane rotor divides the internal space of the housing into retard chambers and advance chambers that are arranged in the rotational direction. By supplying working fluid into the retard chamber or the advance chamber, a rotational phase of the vane rotor relative to the housing (hereinafter, referred merely as a “rotational phase”) is shifted in a retard direction or in an advance direction such that desired valve timing is achieved (see, for example, JP-A-2007-327490 corresponding to U.S. Pat. No. 7,363,897).
The valve timing adjusting apparatus of JP-A-2007-327490 holds the rotational phase at an intermediate position located between the retard end and the advance end of the rotational phase such that the performance of starting the internal combustion engine is sufficiently achieved. Specifically, the valve timing adjusting apparatus of JP-A-2007-327490 has a helical torsion spring having a fixed end that is always engaged with the housing. The other end of the helical torsion spring is a free end. When the rotational phase is in a range on a retard side of the intermediate position, the free end of the helical torsion spring is engaged with the vane rotor such that the vane rotor is urged in the advance direction relative to the housing. Due to the above, at the stopping of the internal combustion engine, until the rotational phase becomes the intermediate position, the vane rotor remains urged by the helical torsion spring in the advance direction, and thereby the vane rotor rotates relative to the housing in the advance direction. As a result, it is possible to hold the rotational phase at the intermediate position during the starting of the internal combustion engine such that the startability of the engine is substantially achieved.
In the valve timing adjusting apparatus of JP-A-2007-327490, the helical torsion spring is located at a position radially outward of a bush that serves as a rotational shaft of the vane rotor. As described above, when the rotational phase is in the range on the retard side of the intermediate position, the free end of the helical torsion spring is engaged with the vane rotor, and thereby the helical torsion spring urges the vane rotor in the advance direction. In contrast, when the rotational phase is in a range on an advance side of the intermediate position, the free end of the helical torsion spring is engaged with the housing such that the vane rotor is prevented from being urged by the spring.
As above, the fixed end of the helical torsion spring is always engaged with the housing, and the free end of the helical torsion spring is engageable with the vane rotor or the housing. In order to mechanically stabilize the helical torsion spring having the above configuration, the helical torsion spring is brought into point-contact with the bush located on the radially inward of the helical torsion spring such that the helical torsion spring applies load to the bush. As a result, when the vane rotor rotates relative to the housing, the helical torsion spring deforms and also slides on the bush accordingly to the relative rotation of the vane rotor. Therefore, sliding resistance may be generated. More specifically, the sliding resistance is generated in opposite directions when the vane rotor is rotated in the retard direction and in the advance direction relative to the housing. In other words, shifting of the rotational phase in the retard direction and in the advance direction generates the friction applied in the opposite directions. As a result, torque is applied to the vane rotor by the urging force of the helical torsion spring and by the frictional force of the sliding resistance. Thus, the applied torque generates hysteresis that has a great difference between the shift of the rotational phase in the retard direction and in the advance direction as shown in FIG. 17. The above hysteresis may deteriorate the accuracy in adjusting the rotational phase or the valve timing by using working fluid, and thereby needs to be improved.